Towed arrays have been used for some time as listening devices for detecting the presence of underwater sound sources. Such arrays consist of a series of interconnected hydrophones with the requisite electronics encased in a flexible tubular jacket. These arrays may be manufactured in sections of any desired length, such as 50 or 100 feet, which may be connected end to end to produce a much longer array. Such arrays are then towed behind a ship, often at a substantial distance and at moderate speeds to minimize noise related to turbulence from the ship's wake and from velocity effects. So long as the array is being pulled through the water certain longitudinal acceleration and deceleration forces on the array are inevitable, and these forces tend to result in the production of spurious signals from the hydrophones.
Previous hydrophones for towed arrays have dealt with the problem of acceleration-induced spurious signals by placing pairs of hydrophones physically back-to-back to produce a structure in which longitudinal accelerations tend to shorten one element while elongating the other, thus canceling or substantially canceling the spurious acceleration-induced signals. Such hydrophones have typically used transducer elements in the form of hollow cylinders of ceramic piezoelectric material with both outside and inside surfaces exposed to oil and having an orifice or port to permit oil to flow from inside to outside or the reverse for hydrostatic pressure compensation. Such fragile elements, even if encased in oil, are subject to damage from rough handling on deck. It has also been found that the pressure-equalizing orifice introduces an undesirable phase shift into the output of the hydrophones--at least at some frequencies. There is also a problem with spurious signals resulting from transverse accelerations which apparently result from some flexing of the side walls of the element.